The present invention relates to a multimedia communication system for transmitting and receiving audio, video and data in a real-time manner by using the Integrated Services Digital Network (referred to as "ISDN" hereinafter), and particularly to a multipoint video conference system for performing a multicast to a plurality of multimedia terminals.
A video conference system is one of such multimedia communication systems that communicate audio, video and data by the use of the ISDN. A typical video conference system is configured such that video conference equipments located at two different places are one-to-one connected to hold a conference. Such a video conference equipment is disclosed in Japanese Patent Application laid-open (KOKAI) publication No. 5-207452 (U.S. Pat. No. 5,477,546).
In order to realize conference communication among three or more locations by using such video conference equipments, a multipoint control unit (MCU) is needed which is connected to each of a plurality of video conference equipments which take part in the conference. The MCU has a plurality of connection ports, typically four, eight, twelve, or sixteen, and also includes the same number of line control units, which allows connection to the video conference equipments of the number of the ports as the upper limit. This means that, when a big conference is held among fifty or one hundred locations, the MCU which has fifty or one hundred of the line control units is required.
Another system for a multipoint video conference without using the MCU, which is disclosed in Japanese Patent Application laid-open (KOKAI) publication No. 5-22322 (U.S. Pat. No. 5,402,418), is known in which a video conference terminal is connected via two information channels of the ISDN to separate terminals such that a loop network is formed among the video conference terminals, which take part in a conference, circulating video and audio data on the loop among the terminals. This system is suited to television conferences for four or five terminals, and uses a screen which is divided into four sections each assigned to each terminal for displaying, on the sub-screen thereof, video of each member attending the conference. Transfer of the video is realized such that a data storage area of one video frame, which is circulated on the loop network, is divided into four subareas, each of which is assigned to each terminal for writing its video data into the subarea. Each terminal acts to delete the video stored in its subarea of a received video frame, which was written one round before, and then write a new video, which was taken at the terminal, into the subarea to transmit it onto the loop.
In the multipoint video conference system using the loop network, mentioned above, the terminals share a video frame of one screen and delete/add their videos from and into their own subareas of the divided video frame which is circulated on the loop. This system is suitable for video conferences among four or five terminals, but is not configured to be used for a large-scale conference where there are terminals in excess of that number, such as fifty or one hundred.